(i) Field of the Invention
This invention relates to a method and apparatus for corrosion protection by electrochemical potential control of a stainless steel vessel containing a corrosive washing solution. More particularly, it is directed to a method and apparatus for protecting stainless steel bleach plant washer equipment, used in the pulp industry, from corrosion.
(ii) Description of the Prior Art
Stainless steel is known to be generally resistant to corrosion due to a build-up thereon of passive films. However, such passive films are susceptible to localized breakdown, and this susceptibility is greatly enhanced in the presence of specific ions. The filtrate used in bleach plant washer generally contains oxidants, e.g., chlorine (Cl.sub.2), chlorine dioxide (ClO.sub.2), or hypochlorite (OCl.sup.-) together with a considerable amount of chloride ions. Such filtrate can bring about localized passive film failure. The loss of protection leads to various modes of localized corrosion, the principal ones being crevice corrosion and pitting corrosion.
The corrosion environment in chlorine and chlorine dioxide stage bleach plant washers is generally acidic, with a pH in the range of 1 to 7. The most common alloys of construction are 316L and 317L stainless steel. In a number of cases, the corrosion of these washers has been so severe that the replacement washer was constructed from stainless steel with higher molybdenum content, at significantly higher cost.
One procedure heretofore used to control corrosion in such bleach plant washers was the addition of antichlors, e.g., SO.sub.2 or NaOH. Such antichlors as used at present are intended to make the environment conditions in the washer less oxidizing and so less corrosive to the stainless steel washer components. It has been found, however, that under typical chlorine dioxide washing stage conditions, for example, SO.sub.2 additions alone will mitigate but will not completely prevent crevice corrosion in many types of stainless steels.
The corrosion environment in hypochlorite stage bleach washers is generally alkaline, with a pH in the range of 8-11. The most common alloy of construction is 316L stainless steel. It would be preferable to use 304L stainless steel since it is much cheaper than 316L stainless steel. However, 304L stainless steel has been largely unsuccessful for this use because of severe chloride crevice corrosion.
Electrochemical corrosion control would seem to offer a solution to the widespread problem of crevice corrosion of stainless steels in bleach washing stages.
Many patents teach the concept of corrosion control by inducing passivity in the metal by anodic polarization techniques. In such technique, the vessel to be protected against corrosion by a chemical contained therein is anodically polarized with respect to an inert electrode suspended in the corrosive liquid in the vessel. An electric current is then passed between an inert electrode suspended in the corrosive liquid in the vessel. An electric current is then passed between the metallic vessel and the inert cathode so as to maintain the electrical potential of the vessel in the passive region. The necessary electrical potential can be determined by means of an anodic polarization curve, or by controlled potential immersion testing. The passive region can be identified after such tests, thus providing data indicative of the potential range within which the vessel should be maintained in order to attempt to minimize corrosion.
Amongst the prior patents dealing with this technique are: Banks et al., U.S. Pat. Nos. 3,371,023 issued Feb. 7, 1968, 3,375,183 issued Mar. 26, 1968, 3,378,472 issued Apr. 16, 1968, 3,379,629 issued Apr. 23, 1968, and 3,409,526 issued Nov. 5, 1968; Elmore et al., U.S. Pat. No. 1,576,581 issued Mar. 16, 1926; Hoey, U.S. Pat. No. 3,442,779 issued May 6, 1969; Poyser, U.S. Pat. No. 4,018,647 issued Apr. 19, 1977; and Hulthe, U.S. Pat. No. 4,036,716 issued July 19, 1977.
Cathodic polarization has also been applied in the past for the protection of metals that do not form passive films, e.g., iron in soil or sea water. It has been applied to stainless steels in neutral sea water solutions, but its application to stainless steels in strongly oxidizing chloride solutions involves a number of possible difficulties mainly concerned with the problem of not destroying the passive film when the potential is lowered to a potential near or below the protection potential. Precise control of the impressed current in a cathodic system is thus a prime requirement.
Amongst the prior patents which relate to the application of cathodic protection of metal surfaces including such precise control of the impressed current are MacTaggart et al., U.S. Pat. No. 2,435,973 issued Feb. 17, 1948; Stephens, Jr., U.S. Pat. No. 3,634,222 issued Jan. 11, 1972; Kipps et al., U.S. Pat. No. 3,692,650 issued Sept. 19, 1972; and Ferry et al., U.S. Pat. No. 4,080,272 issued Mar. 21, 1978.
However, it has been found that, in oxidizing aqueous chloride environments, stainless steels are most susceptible to crevice corrosion. While it is known that crevice corrosion can be arrested by cathodic polarization, that is, by impressing a current between the steel to be protected, and an inert electrode, so that the steel is cathodic with respect to that electrode, it is nevertheless true that if stainless steel is maintained at a cathodic potential in this way, it may undergo general dissolution due to the reduction of the thin surface oxide film which gives the steel its stainless property. Thus, as practiced in the past, cathodic polarization may arrest crevice corrosion but may cause general attack of stainless steel in strongly oxidizing chloride solutions.
Another difficulty in cathodic polarization of stainless steel bleach plant washers is the placement of the anode within the washer. Since the washer consists of a vat containing the bleached pulp to be washed and a perforated or foraminous stainless steel washing drum rotatably mounted within the tank to provide the wash water, a considerable amount of shear force is generated when the drum is rotated which may dislodge the anode.